Carbon dioxide is a greenhouse gas. The more of it there is in the atmosphere, the more heat will be retained, leading to warming. This has been known since the 19th century, and this is true though the proportion of CO2 in the atmosphere is relatively tiny, now four hundred parts per million, and though plants require to breathe CO2 to survive.
Burning fossil fuels produces CO2. Heat is released when the fossil fuel reacts with oxygen: for example, methane, CH4, plus O2 produces H2O plus CO2. Children are taught this at different ages, but a fifteen year old who will never be capable of a university degree will be capable of understanding this.
Volcanoes produce CO2, and a lake in Cameroon belched forth a vast quantity of it, killing all animal life nearby; but the amount produced by industry is orders of magnitude greater.
The Earth’s climate and atmospheric CO2 levels, have changed in the past. However, now, both are changing catastrophically quickly, with results such as increased bleaching of coral, species migrating to cooler habitat closer to the poles or higher up mountains, alterations in climate such as rainfall patterns, and species extinction.
Sea levels will rise as ice on land melts and warming water expands.
Most people are capable of understanding these facts, but some wilfully deny or obfuscate them.
There is a great deal of doubt about how the world will be affected in the future. How quickly sea level will rise and how climate round the world will change are uncertain, though scientists get better at making predictions as more data emerges. However, the basic facts, showing that climate change is a threat, are clear.
Burning fossil fuels has been an abundant source of cheap energy, fuelling the Industrial Revolution and producing a huge improvement in human living standards. Making the equipment to get energy from other sources, such as solar, wind and tide, has an environmental cost, and often that energy is more expensive than fossil fuel energy, especially if environmental degradation is not factored in as a cost. Obtaining energy in different ways causes other environmental damage, such as pollution, as well as CO2 release. There are complex political and economic arguments to be made, and for these decisions we need the best possible data.
There are political and economic arguments that humanity should continue burning fossil fuels for the energy. I would have more confidence in them if those arguing for fossil fuel use did not feel driven to lie about the basic facts.
There are climate-change denialists, who would claim there is doubt about the basic facts, or minimise the damage that fossil fuel use will do. Human beings are often selfish and irrational, failing to defer gratification, or denying facts which they find uncomfortable. However highly paid and educated people, or politicians, should be able to acknowledge the basic facts.